WCDMA (Wideband Code Division Multiple Access) wireless communication systems employ a direct code spreading multiplex process. According to the direct code spreading multiplex process, the transmission side spreads transmission data with a spreading code, and the reception side despreads reception data with the same spreading code. The reception data thus processed have a higher ratio of desired wave power to interference and noise power (SNIR: Signal to Noise Interference Ratio).
On the reception side, if the SNIR based on the despreading process is equal to or higher than a predetermined value, i.e., if the reception data have a predetermined quality or higher, then desired reception data can be decoded properly. Consequently, even when a plurality of links use the same frequency band, the above data spreading and despreading process makes it possible for the reception side to decode the reception data of the respective links.
Generally, as the spreading ratio on the transmission side is lower, the number of information bits that can be transmitted within the same time is greater and the transmission ratio is higher. On the other hand, because the increase in the SNIR based on the despreading process is reduced, it is necessary to increase the transmission power in order to satisfy the predetermined quality.
According to the direct code spreading multiplex process, the transmission power of a certain link serves as the interference power of another link. Therefore, it is important to set a transmission rate which can minimize the transmission power while satisfying transmission rate requirements, in each link for reducing interference to other links, and such a transmission rate leads to a reduction in the bandwidth of the wireless communication system.
Consequently, the WCDMA wireless communication system controls the transmission power of mobile and base stations to achieve transmission data of predetermined quality under high-speed closed-loop transmission power control.
According to 3GPP (3rd Generation Partnership Project), it has been studied to give a function to select a Transport Format Combination (hereinafter referred to as “TFC”) to a WCDMA mobile station (see 3GPP TS 25.321 V5.8.0 (2004-March) “Medium Access Control (MAC) protocol specification”).
The WCDMA mobile station is capable of transmitting the data of a plurality of different transport channels through a single physical channel. The mobile station generally uses a DPCH (Dedicated Physical Channel) as a physical channel. The DPCH includes a DPCCH (Dedicated Physical Control Channel) for sending pilot data and control data and a DPDCH (Dedicated Physical Data Channel) for sending user data. A transmission type called a Transport Format (hereinafter referred to as “TF”) is set in each transport channel. Setting items of the TF include a transport block size, a CRC (Cyclic Redundancy Check) bit size, a coding process, a Transmission Time Interval (TTI), etc. The TFC referred to above represents a combination of TFCs set in difference transport channels.
According to the WCDMA wireless communication system, a base station control device indicates a TFC set including one or more TFCs to be permitted for the physical channel of each mobile station, and a mobile station selects a TFC to be used for sending a DPCH from the TFC set indicated by the base station control device.
A process of determining a state of a mobile station when it uses a TFC will be described below with reference to FIG. 1.
First, the transmission power of a DPCH when a TFC is used is calculated with respect to each TFC.
Then, the states of the mobile station when the respective TFCs are used are all classified as a support state.
If, among the TFCs belonging to the support state, there is a TFC wherein the transmission power of the DPCH is greater than the maximum transmission power of the mobile station for a time Y or longer in a predetermined time X in the past, then the state of the mobile station which uses that TFC is regarded as an excess power state.
If, among the TFCs belonging to the excess power state, there is a TFC which belongs to the excess power state for a predetermined time T or longer, then the state of the mobile station which uses that TFC is regarded as a block state.
If, among the TFCs belonging to the excess power state or the block state, there is a TFC which wherein the transmission power of the DPCH is equal to or smaller than the maximum transmission power of the mobile station continuously for a predetermined time Z, then the state of the mobile station which uses that TFC is returned to the support state.
The mobile station determines the state of the mobile station when it uses the TFCs, according to the above process. The mobile station selects a TFC for setting a TF with a high transmission rate in a transport channel with high priority, from the TFCs in other than the block state. Since the state of the mobile station when it uses the TFCs is determined based on long-term propagation path variations, a TFC which satisfies quality requirements on the average over a long time can be selected even if the propagation path varies instantaneously due to fading or the like.
At present, it is studied according the 3GPP to use an EUDCH (Enhanced Uplink Data Channel) as a physical channel for transmitting packets at a high speed through an uplink. With the EUDCH, it is studied that the base station and the base station control device will control the packet transmission format (primarily the transmission rate) for the uplink of a mobile station, using the TFC selecting function of the mobile station (see 3GPP TR 25.896 V6.0.0 (2004-March) “Feasibility Study for Enhanced Uplink for UTRAFDD”).
The study shows that in the WCDMA wireless communication system, a base station measures the proportion (noise rise) of noise power in a desired wave of data received from a mobile station, and a base station control device controls the number of mobile stations connected to the base station and TFC sets in the mobile stations so that the above value measured by the base station will not exceed a predetermined threshold value.
Generally, however, the data transmission between the base station and the base station control device suffers a certain delay, and the data transmission from the base station control device to the base station also suffers a large delay. Therefore, it is difficult for the base station control device to control the number of mobile stations and the TFC sets depending on instantaneous noise rise changes.
Consequently, the conventional WCDMA wireless communication system has been required to set the number of mobile stations and the TFC sets in order to keep an average noise rise value sufficiently smaller than predetermined threshold value, thereby providing against sharp noise rise changes.
With the EUDCH, it is studied that the base station will indicate a TFC (maximum TFC) wherein the transmission power of an EUDCH is maximum, among TFCs that are allowed to be used, at a high speed to a mobile station, and the mobile station will select a TFC wherein the transmission power of an EUDCH is equal to or smaller than the transmission power of the EUDCH in the case where the maximum TFC indicated by the base station is used.
Since the above study makes it possible to reduce a variable noise rise range, the average noise rise value can be set to a higher level. In other words, since the number of mobile stations connected to the base station and the maximum power value of the maximum TFC can be set to higher values than heretofore, the coverage and capacity of the uplink are increased.
However, because the mobile station uses not only an EUDCH but also a DPCH referred to above, the mobile station needs to select a TFC also for the DPCH. Therefore, the mobile station has to select two TFCs, i.e., a TFC for the EUDCH and a TFC for the DPCH.
As described above, the mobile station determines the state of the mobile station for selecting TFCs based on whether the transmission power consumed when each TFC is used is greater than the maximum transmission power of the mobile station or not.
As shown in FIG. 2, for example, when the mobile station is to select a TFC for an EUDCH (hereinafter referred to as “E-TFC”), the mobile station may select an E-TFC4 wherein the transmission power of an EUDCH is represented by PEUDCH which smaller than the maximum transmission power Pmax, and when the mobile station is to select a TFC for a DPCH, the mobile station may select a TFC6 wherein the transmission power of a DPCH is represented by PPDCH which is smaller than the maximum transmission power Pmax.
However, when the mobile station transmits data simultaneously in the EUDCH and the DPCH, the sum (PEUDCH+PPDCH) of the transmission power of the EUDCH and the transmission power of the DPCH exceeds the maximum transmission power Pmax of the mobile station, giving rise to a problem in that the mobile station suffers a shortage of transmission power.
In this case, it is necessary to reduce the transmission power of either one or both of the TFC and the E-TFC to make a power adjustment for reducing the total transmission power to a level equal to or smaller than the maximum transmission power Pmax. However, another problem occurs in that the quality of data transmitted through the physical channel whose transmission power has been reduced is deteriorated.